Seismic techniques are used today to explore the oceans of the earth for deposits of oil, gas, and other minerals. In such exploration, an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air gun. The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back to a streamer towed behind the vessel where it is detected by hydrophone sensors in the streamer to yield data regarding characteristics of the subsurface geologic structures. Land seismic systems use geophones on land in a similar manner. Ocean bottom and transition zone systems use both hydrophones and geophones.
The towed streamer includes a large number of pressure-sensitive hydrophone sensors enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone sensor within the streamer is designed to convert pressure variations surrounding the hydrophone sensor into electrical signals. Due to its extreme length, which may be thousands of meters, the streamer is often divided into a number of separate sections that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone sensors. Data busses, running through each of the sections of the streamer, carry the signals in the form of acoustic data from the hydrophone sensors to the recording equipment.
In addition to acoustic data, the streamer collects and transmits data concerning operational status of the array to the vessel. This data is referred to as non-acoustic data. This data comprises physical characteristics of interest regarding the operation of each section, including whether water has invaded a field acquisition unit (AU) or module in the streamer, temperature, depth, and power supply voltage for components in the streamer.
Many towed arrays have digital data channels. With digital data transmission, data transmission rates are higher and, with proper attention to electromagnetic interference, data fidelity is maintained from the hydrophones to the recording equipment.
Typically, AUs are designed with fixed numbers of acquisition channels and a number of additional information channels for AU status. A channel is used to acquire the acoustic data from a group of hydrophones or geophones at or near one physical point in the streamer or cable space. For example, Syntron's streamer module or AU has 12 channels. Each system typically transmits down a single link that connects a recorder and a number of AUs and the recorder and an AU may be thousands of meters apart. The number of AUs may be one hundred or so and the distance separating them may be on the order of hundreds of meters.
In each system, two of the limiting factors in maximum telemetry length and channel count are the distance between AUs (to repeat data) and the number of AUs whose data will fill the time slots (at fixed transmission rates). The data packet from each unit typically has a fixed length in terms of number of bits and therefore takes a fixed time to send at the maximum transmission rate (measured in bits per second). Since the data must be transmitted in real time and a certain sampling interval (typically 1, 2, or 4 ms) is specified, the number of AUs is limited, limiting maximum telemetry length and channel count.
The number of channels per AU differs in land, marine, ocean bottom, and transition zone applications for many reasons, ranging from a high of sixteen to a low of one channel in known systems. The reasons for the variation in the number of channels per AU include the length of sensor intervals, complexity, AU size, weight, and power, cable complexity, length, and weight, and deployment site variables such as ravines, streams and water zones, and accessibility. It is expected that some future systems will continue to include more and more channels per AU and that some will have small numbers of channels where conditions warrant.
In most present systems, all AUs in the system have the same fixed number of channels. In one exception, one known system can be configured with one, three, or six channels per AU, but in that system, transmission length is fixed with a specific hardware configuration and all units must have the same channel configuration.
The present invention is part of a variable channel AU which may include from 4 to 256 physical channels, the number of channels being transmitted a function of both physical capacity of the specific AU and remote programming of the AU. It is intended to activate only up to the number of channels used, which may be fewer than the number actually installed. Remote programming is accomplished by means of sending commands to the AU over a command data bus originating in the seismic system central recorder.
Other systems could turn off data transmission from AUs just acting as repeaters so that there would be more telemetry time for units more remote from the towing vessel. Such systems, however, suffer in that there is no information emanating from those repeater AUs, thus making telemetry problems difficult to troubleshoot. It is desirable to know the internal conditions (like temperature, pressure, operating voltages and currents, clock errors, etc.) of units merely acting as data repeaters because errors in them can adversely affect the data.
In Syntron's present system, the Syntrak, a 12-channel AU, data reduction to six channels (by means of summing or array forming channels) still results in a full data packet (enough to handle all 12 channels) rather than reducing the packet size.
Because early seismic systems usually were composed of acquisition units that were the same, early seismic systems handled only fixed length seismic packets. In fact the earliest ones only had one channel per unit anyway. Circuits that handled seismic unit data were kept simple in having to deal with fixed format packets, each system having unique packet formats.
Thus, there remains a need for a seismic system which may be adapted to include a variable number of channels per acquisition unit and which therefore includes a variable length of acoustic data packet to accommodate the variation in the number of channels.
The seismic system central recorder must have corresponding ability to decode incoming data packets, determine the number of active channels for each AU, and recover the channels' data from the packet.
By having a telemetry protocol in which the number of included channels is defined, such a system provides the following advantages:
(1) All devices using the protocol may talk to a common data receiver unit in the central seismic recorder that is an integral part of all seismic acquisition systems, without having to change the design of the receiver. PA1 (2) The system provides more efficient use of bandwidth. A large number of small-channel-count AUs may be used, a smaller number of large-channel-count AUs may be used, or a mixture of large-channel-count and small-channel-count AUs may be intermixed on a telemetry line. PA1 (3) Rapid engineering of new AUs of specific maximum channel-counts for specific application to land, jungle, transition zone, marine streamers, single-component ocean bottom, dual-component ocean bottom, or four-component ocean bottom uses, in various combinations of group interval lengths is possible. PA1 (4) Such a system provides the ability to retrieve continuous state-of-health status from units programmed for zero channels (data repeaters). PA1 (5) Tailoring of AUs to meet customer cost goals by adding or deleting acquisition channels, the most costly components, from AU chassis, to meet specific channel requirements, is provided.